Comparing Ultra-hypofractionated Proton versus Photon Therapy in Extremity Soft Tissue Sarcoma

Abstract

Purpose Recent single institution, phase II evidence has demonstrated the feasibility and efficacy of ultra-hypofractionated, preoperative photon therapy in 5 fractions for the treatment of soft tissue sarcoma (STS). Our purpose was to evaluate the dosimetric benefits of modern scanning beam proton therapy compared with conventional photon radiation therapy (RT) for the neoadjuvant treatment of adult extremity STS. Materials and Methods Existing proton and photon plans for 11 adult patients with STS of the lower extremities previously treated preoperatively with neoadjuvant RT at our center were used to create proton therapy plans using Raystation Treatment Planning System v10.A. Volumes were delineated, and doses reported consistent with International Commission on Radiation Units and Measurements reports 50, 62, and 78. Target volumes were optimized such that 100% clinical target volume (CTV) was covered by 99% of the prescription dose. The prescribed dose was 30 Gy for PT and RT delivered in 5 fractions. For proton therapy, doses are reported in GyRBE = 1.1 Gy. The constraints for adjacent organs at risk (OARs) within 1 cm of the CTV were the following: femur V30Gy ≤ 50%, joint V30Gy < 50%, femoral head V30Gy ≤ 5 cm3, strip V12 ≤ 10%, and skin V12 < 50%. Target coverage goals, OAR constraints, and integral dose were compared by Student t test with P < .05 significance. Results A minimum 99% CTV coverage was achieved for all plans. OAR dose constraints were achieved for all proton and photon plans; however, mean doses to the femur (10.7 ± 8.5 vs 16.1 ± 7.7 GyRBE), femoral head (2.0 ± 4.4 vs 3.6 ± 6.4 GyRBE), and proximal joint (1.8 ± 2.4 vs 3.5 ± 4.4 GyRBE) were all significantly lower with PT vs intensity-modulated radiation therapy (IMRT) (all P < .05). Integral dose was significantly reduced for proton vs photon plans. Conformity and heterogeneity indices were significantly better for proton therapy. Conclusion Proton therapy maintained target coverage while significantly reducing integral and mean doses to the proximal organs at risk compared with RT. Further prospective investigation is warranted to validate these findings and potential benefit in the management of adult STS.

Team Approach: Extremity Soft Tissue Sarcoma

“…These studies reported the safety and efficacy of this approach and established the currently advised planning margins that would be used in this case—specifically, 3 to 4 cm superiorly/inferiorly and 1.5 cm radially, cropping the clinical target volume extending beyond the muscular compartment and including the suspicious edema and guided by the use of CT-based planning fused with T1 postcontrast and T2 MRI sequences to delineate the tumor volume 15 . Nonetheless, in the aforementioned IGRT trials, wound healing and/or acute toxicity complication profiles were generally similar to those of earlier approaches; techniques to improve toxicity profiles through dose-reduction are thus being pursued 20,21 . The standard, conventional preoperative dose in this case would be 1.8 to 2 Gy in 25 to 28 daily (weekday) fractions of external beam radiation (Fig.…”

Prospective phase II trial of preoperative hypofractionated proton therapy for extremity and truncal soft tissue sarcoma: the PRONTO study rationale and design

“…To explore this potential risk, our group conducted a dosimetric analysis, comparing target coverage and dose to surrounding organs at risk when treating extremity STS with identical hypofractionated regimens of photon and proton radiation as those which will be used in this trial [ 26 ]. Doses to skin were statistically similar between the two modalities for most of the dosimetric endpoints outside of mean and maximum doses, which were slightly lower with PBT than photon RT.…”

Prospective Phase II Trial of Preoperative Hypofractionated Proton Therapy for Extremity and Truncal Soft Tissue Sarcoma: the PRONTO Study Rationale and Design

“…To explore this potential risk, our group conducted a dosimetric analysis, comparing target coverage and dose to surrounding organs at risk when treating extremity STS with identical hypofractionated regimens of photon and proton radiation as those which will be used in this trial (25). Doses to skin were statistically similar between the two modalities for most of the dosimetric endpoints outside of mean and maximum doses, which were slightly lower with PBT than photon RT.…”

“…As shown, the target receives similar coverage in the two plans, while the normal tissue outside of the target receives lower doses in the proton plan. Figure adopted from study by Thomas et al (25).…”